The invention relates to a spherical aberration control method and related apparatus, and more particularly to a spherical aberration control method that adjusts the spherical aberration of an optical device according to temperature variation, and a related apparatus.
With increasing demands for larger storage capacity, traditional CDs and DVDs can no longer satisfy user requirements. Blu-ray discs (BD) and High Density Digital Versatile Discs (HD-DVD) providing large data storage size are becoming the future of optical storage. The numerical aperture (NA) of an object lens used in a BD optical disc driver and an HD-DVD optical disc driver is 0.85 and 0.65 respectively, both apertures being significantly larger than the 0.45 NA object lens of a traditional CD optical disc driver. Also, BDs and the HD-DVDs have multi-layer structures and can thus provide larger storage capacity than conventional optical storage.
Certain major aberrations in an optical system are spherical aberration, coma aberration, astigmatism, and curvature of field. A lens is theoretically capable of focusing light onto a single point. The light spot is required to focus on a specific surface on an optical storage medium. However, spherical aberration causes that the lens fails to focus incident light onto a single point. The erroneous focusing of the lens can be caused by the thickness variation of an optical storage medium, in which the laser light could not focus onto the required surface of the storage medium for data access. The erroneous focusing may also result from a zonal distribution of the focusing position (more particularly in the direction of light). Another reason is that the thickness or surface curvature of the lens varies with temperature, expanding when hot and shrinking when cold, which will also cause the failure of the laser light to focus onto the required surface of the storage medium for data access.